Difficultly biodegradable chelating agents, particularly aminopolycarboxylic acids, are being popularly used in various fields such as industrial soaps, the photographic industry, the pulp industry and the plating industry. It has been pointed out that, in addition to the difficultly biodegradable properties, the chelating agents contained in the industrial wastewater discharged from these fields might possibly enter into surface layer water, specifically rivers, and ground water. For example, in Germany, discharge of the difficultly biodegradable chelating agents of aminopolycarboxylic acids is self-imposed. In Japan, there exist at present no regulations on use and discharge of them but, since the difficultly biodegradable aminopolycarboxylic acids increase the COD value of a wastewater, the COD value of the wastewater containing the difficultly biodegradable aminopolycarboxylic acids has become problematical in view of regulation of COD.
As methods for treating difficultly biodegradable wastewater such as a wastewater containing an aminopolycarboxylic acid for reducing a COD load of the wastewater, there have conventionally been known a chemically treating method (JP-B-57-37396, JP-A-61-241746, etc.), a reverse osmosis method (JP-A-50-22463), an activated sludge method (JP-B-55-49559, JP-B-51-12943, etc.), an electrolytic oxidation method (JP-A-48-84462, JP-A-49-119458, etc.), etc. However, these methods respectively have the following defects.
As the chemically treating method, there have been known treating methods by adding hydrogen peroxide, persulfates, perhalogenates, chlorous acid, or hypochlorous acid. For wastewaters having a high COD (chemical oxygen demand) value, however, all of the methods show such a poor treating efficiency that chemicals are used always in amounts more than is necessary, leading to a high operation cost. In the case of using a membrane as with the reverse osmosis method, the membrane must be exchanged so frequently due to adsorption of the components in the wastewater or due to dirt that the operation cost is liable to become high.
As to the activated sludge method, its operation cost is inexpensive, but its effect on materials having a poor biodegradability is low. Particularly, it is almost ineffective for treating a wastewater containing an aminopolycarboxylic acid such as EDTA (ethylenediaminetetraacetic acid).
Although the electrolytic oxidation method shows a strong oxidation power, it involves such problems as (1) that oxidative decomposition treatment of a waste liquor having a high COD value requires a large electric current and a high cost of equipment and takes a prolonged treating time and (2) that decomposition of organic compounds often stops at a level of lower fatty acids such as propionic acid which have a load of BOD (biological oxygen demand), leading to an insufficient decrease of the BOD value.
There have been disclosed methods of carrying out electrolytic oxidation under high-speed agitation as means to further enhance the effect of electrolytic oxidation; for example, in JP-A-8-281272 is set forth a wastewater-treating apparatus to which high-speed electrolysis is applied, in JP-A-9-40482 is set forth a method of obtaining an aqueous manure solution from the effluent from electroless plating and an apparatus for such method, and in WO 02/090621 is set forth a method of electrolytically treating wastewater in continuous mode and an apparatus for such method. However, no one of these disclosed patent literatures contains any description on the treatment of aminopolycarboxylic acid, and in addition the preferable pH range is adjusted not to exceed 6.
It is also possible to conduct the treatment of wastewater according to an ozone decomposition method, but its large cost is a bottleneck.
These difficultly biodegradable compounds are scarcely decomposed by the conventional activated sludge treatment, and the alternative methods are not effective as described hereinbefore. Hence, a dilution method of diluting a wastewater before its discharge is the most general method for decreasing the COD value to a regulation level or lower. However, the dilution method is costly and fails to reduce the total amount of discharged COD. Thus, there has been desired an essentially problem-solving means of substantially decreasing COD in the discharged water.